1. Field of the Invention
The present invention relates to a method of fabricating a semiconductor device using a crystalline semiconductor film formed by crystallizing an amorphous semiconductor thin film, and particularly to a method of improving reliability of a semiconductor device. The semiconductor device of the present invention includes not only a component such as a thin film transistor (TFT) or a MOS transistor but also an electro-optical device such as a display device or an image sensor including a semiconductor circuit constituted by such insulated gate transistors. In addition, the semiconductor device of the present invention includes an electronic equipment incorporating such a display device and an electro-optical device.
2. Description of the Related Art
In recent years, a technique for constructing a semiconductor circuit by forming TFTs on a glass substrate or the like has been rapidly developed. As such a semiconductor circuit, an electro-optical device such as an active matrix type liquid crystal display device is typical.
The active matrix type liquid crystal display device is a monolithic display device in which a pixel matrix circuit and a driver circuit are provided on the same substrate. Moreover, development of a system-on-panel having a built-in logic circuit such as a memory circuit or a clock generating circuit has also been made.
Since the driver circuit or the logic circuit of the active matrix type liquid crystal display device is required to perform a high speed operation, it is unsuitable to use a noncrystalline silicon film (amorphous silicon film) as an active layer. Thus, under the present circumstances, a TFT having an active layer of a crystalline silicon film (polysilicon) has become the mainstream.
Since a TFT can be formed on a transparent glass substrate, application and development to an active matrix type display device has been actively made. Since a TFT using a polysilicon film can obtain a high mobility, it is possible to realize image display of high fineness by integrating functional circuits on the same substrate.
In the active matrix type display device, as the resolution of a screen becomes high, one million TFTs become necessary even for only pixels. Further, if a functional circuit is added, more than one million TFTs becomes necessary. In order to make the liquid crystal display device operate stably, it has been necessary to secure reliability of individual TFTs and to make them operate stably.
In such an active matrix type display device, especially in the case where a TFT is provided on a substrate (for example, a glass substrate) having bad thermal conductivity and good heat retaining property, since a large voltage and current is applied to a TFT of a peripheral driver circuit, a semiconductor layer generates heat and remarkably lowers the reliability of the TFT.